Processes for selectively reducing the concentration of hydrogen cyanide in syngas

ABSTRACT

This invention pertains to processes for selectively oxidizing hydrogen cyanide contained in syngas using permanganate anion as an oxidant contained in an aqueous solution that is contacted with the syngas under certain conditions of temperature, pressure and duration of contact.

CROSS REFERENCE TO RELATED PATENT APPLICATIONS

The present application is a continuation of U.S. patent Ser. No.14/079,335, now U.S. Pat. No. 8,974,759, filed Nov. 13, 2013, which is acontinuation-in-part of U.S. patent Ser. No. 13/304,902, now U.S. Pat.No. 8,895,274, filed on Nov. 28, 2011, each being incorporated herein intheir entirety by reference.

FIELD OF THE INVENTION

This invention pertains to processes for selectively reducing theconcentration of hydrogen cyanide in syngas, and particularly toprocesses that enable permanganate anion to selectively oxidize hydrogencyanide.

BACKGROUND

Numerous proposals exist for producing gases containing carbon monoxide,hydrogen and, optionally, carbon dioxide. For purposes herein, thesegases are referred to as synthesis gas (syngas). Syngas can be producedfrom virtually any carbon-containing feedstock including, but notlimited to, biomass and fossil fuels such as natural gas, petroleum andcoal. Processes for generating syngas include, but are not limited to,gasification, partial oxidation and reforming (autothermal and steam).Other sources of the gas substrate include gases generated duringpetroleum and petrochemical processing and off-gases from manufacturingoperations including, but not limited to, blast furnace and coke ovenoperations, the steel industry, non-ferrous metal industries, orcaptured gas from incomplete combustion processes.

Syngas can be a fuel but is often used as an intermediate for theproduction of other chemicals such as ammonia, methanol, and syntheticpetroleum via the Fischer-Tropsch process. Syngas can also bebioconverted to produce alkanols, diols, carboxylic acids and esters,and alkanes such as methane.

Impurities in syngas generally exist due to the source of the syngas andthe raw materials used to generate the syngas. Examples of impuritiesthat can exist in syngas include, but are not limited to, acetylene,ethylene, hydrogen sulfide, carbonyl sulfide, carbon disulfide, nitricoxide, and hydrogen cyanide. Some of these impurities can be deleteriousto the intended application of the syngas and accordingly must beremoved. In bioprocesses, certain impurities can be inhibitors to one ormore metabolic pathways or can be lethal to the microorganism. See, forinstance, Xu, et al., The Effects of Syngas Impurities on SyngasFermentation to Liquid Fuels, Biomass and Bioenergy, 35 (2011),2690-2696; United States Published Patent Application No. 20110097701;Abubackar, et al., Biological Conversion of Carbon Monoxide: Rich Syngasor Waste Gases to Bioethanol, Biofuels, Bioproducts & Biorefining, 5,(2011), 93-114; and Munasinghe, et al., Biomass-derived SyngasFermentation into Biofuels: Opportunities and Challenges, BioresourceTechnology, 101, (2011), 5013-5022. Particularly deleterious impuritiesto microorganisms are hydrogen cyanide and acetylene. Hydrogen cyanideis also deleterious to catalysts used for Fischer-Tropsch processes.

U.S. Pat. No. 4,189,307 discloses a process for the removal of hydrogencyanide from syngas by absorption in water. European Patent 1 051 351 B1discloses processes for the removal of ammonia and hydrogen cyanide fromsyngas. In the disclosed process, hydrogen cyanide is converted toammonia and is removed from the gas with water. A hydrocarbon gas usedas the feedstock to the synthesis gas generator is used to strip ammoniaout of the water. A catalytic hydrolysis or hydrogenation is used toconvert the hydrogen cyanide to ammonia. It is stated at page 3 that “ .. . the concentration of the combined total hydrogen and ammonia presentin the syngas is preferably reduced to less than 0.1 vppm . . . ” Thedisclosed process is complex and requires a catalyst which may bedeactivated. Indeed, the patent discloses at column 5, lines 4 et seq.,the use of substantially sulfur free methane.

U.S. Pat. No. 8,303,849 discloses processes for removing hydrogencyanide from syngas using chemical and biological treatment. The processuses an aqueous scrubbing unit operation under certain conditions andthen subjects the aqueous solution to microbial activity to degrade thehydrogen cyanide.

Processes are still sought to remove hydrogen cyanide from syngas,especially to concentrations of less than 1 part per million by volumein an economically attractive manner. More desirably, such processeswould degrade hydrogen cyanide. Even further advantageous processeswould remove other contaminants from the syngas.

SUMMARY OF THE INVENTION

It has been surprisingly found by this invention that permanganateanion, under certain conditions, can selectively oxidize hydrogencyanide and certain other contaminants in syngas with negligibleoxidation of carbon monoxide and hydrogen. The selective oxidation ofhydrogen cyanide can be effected even at its typically very lowconcentrations in the syngas, to a concentration of below 1, preferablybelow 0.5 or even 0.1, part per million by volume. Yet, carbon monoxideor hydrogen, which are often in combined concentrations greater than 50volume percent of the syngas, are only negligibly oxidized even whenhydrogen cyanide is reduced to such low concentrations in the gas. Theprocesses are particularly unexpected in that permanganate anion isknown as an oxidant of carbon monoxide. See, for instance, United StatesPatent Application Publication No. 2012/0009109 which states atparagraph [0062] that “The CO oxidation catalyst may include variouspermanganate salts, e.g., silver permanganate and potassiumpermanganate, impregnated on a suitable support such as alumina; zincoxide; silica; zeolite; titania; and zirconia.”

It is found by this invention that a dilute solution of permanganateanion in an aqueous solution can effectively oxidize hydrogen cyanide,which hydrogen cyanide may be present in very low concentrations in thesyngas, to very low concentrations, e.g., less than 1 part per millionby volume, by maintaining the aqueous solution within certain pressureand temperature ranges. In its broad aspects, the processes of thisinvention for selectively reducing the concentration of hydrogen cyanidein a feed gas containing at least about 5 volume percent carbon monoxideand between about 1 and 500 parts per million by volume hydrogencyanide, comprise:

-   -   a. continuously passing said gas feed into contact with an        aqueous solution containing between about 50 and 2000,        preferably less than about 1000, and often between about 100 and        750, parts per million by mass of permanganate anion under        conditions sufficient for permanganate anion to oxidize hydrogen        cyanide and generate manganese dioxide,    -   i. said contacting being at a temperature of between about 4° C.        and 50° C., preferably between about 10° C. and 40° C.,        -   ii. said gas feed being at a pressure less than about 2000            kPa absolute, and often between about 105 and 1500 kPa, and            in some instances between about 110 and 150 kPa, and        -   iii. wherein the duration of said contacting is at least            sufficient to provide a treated gas having a concentration            of hydrogen cyanide that is less than about 30 percent of            that in the gas feed, and preferably less than about 1, more            preferably less than about 0.5, and most preferably less            than about 0.1, part per million by volume hydrogen cyanide;    -   b. continuously removing said treated gas from contact with the        aqueous solution;    -   c. intermittently or continuously replenishing permanganate        anion to said aqueous solution; and    -   d. intermittently or continuously removing manganese dioxide        from said aqueous solution.

In a preferred aspect of the invention, the parameters of temperature,pressure and duration of contact between the gas feed and the aqueoussolution are such that the rate of conversion of permanganate anion perunit volume of aqueous solution to manganese dioxide is maintained at alevel where the desired reduction in hydrogen cyanide concentration isobtained and the oxidation is selective. Often, less than about 100,preferably less than about 75, parts per million by mass of permanganateanion of aqueous solution are converted to manganese dioxide per second.Preferably, the duration of contact between the gas feed in an aqueoussolution is less than about 30, and more frequently less than about 10,seconds.

In preferred embodiments the parameters of temperature, pressure andconcentration of permanganate anion are such that the oxidation ofhydrogen cyanide is mass transfer limited, i.e., the rate of oxidationof hydrogen cyanide in the aqueous solution is more rapid than the rateof mass transfer of hydrogen cyanide from the gas feed to the aqueoussolution. The operation in a mass transfer-limited mode provides severaladvantages to the processes of this invention. First, the concentrationof hydrogen cyanide in the aqueous solution remains low therebyenhancing the driving force for the mass transfer of hydrogen cyanidefrom the gas phase to the aqueous solution. Thus the contact timebetween the gas and aqueous phased is minimized for a given mass ofhydrogen cyanide to be removed from the gas phase. Second, thelimitation of the mass transfer of hydrogen cyanide facilitatescontrolling the rate of conversion of permanganate anion per unit volumeaqueous solution to manganese dioxide.

In further preferred embodiments of the processes of this invention, thecontact between the gas feed and aqueous solution is conducted in anabsorption vessel, and aqueous solution is continuously added andwithdrawn from the absorption vessel to maintain a steady state,continuous operation. Preferably, the flows of the gas feed and of theaqueous solution are countercurrent. Typically, the withdrawn aqueoussolution is recycled to the absorption vessel. In order to reduce therate of degradation of permanganate, which is catalyzed by the presenceof manganese dioxide, manganese dioxide is removed from all or a portionof the aqueous solution being recycled. Manganese dioxide is a densesolid and can be removed by any suitable solids removal unit operation.Makeup permanganate anion and water can be added to the aqueous solutionrecycle.

An additional advantage of the processes of this invention is thatcertain other components contained in the gas can be selectivelyoxidized with negligible oxidation of carbon monoxide and hydrogen.These components include, but are not limited to, acetylene, nitricoxide and hydrogen sulfide. Acetylene and nitric oxide can adverselyaffect microorganisms. Interestingly, hydrogen cyanide, acetylene,nitric oxide and carbon monoxide are characterized as having triplebonds, yet carbon monoxide, despite being present in multiple magnitudesof higher concentration, is negligibly oxidized in the processes of thisinvention.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic depiction of an apparatus for practicing a processof this invention to remove hydrogen cyanide from syngas.

DETAILED DISCUSSION

Definitions

As used herein, the following terms have the meanings set forth belowunless otherwise stated or clear from the context of their use.

The use of the terms “a” and “an” is intended to include one or more ofthe element described, and unless explicit or otherwise clear from thecontext, an element recited in the singular is intended to include oneor more of such elements.

The term Component Composition means the composition of a gas where bothwater and nitrogen, although they may be present in the gas, have beenexcluded from the calculation of the concentration of the components. Asused herein, unless otherwise stated, compositions of gases are on ananhydrous basis and exclude the presence of nitrogen.

Intermittently means from time to time and may be at regular orirregular time intervals.

Oxygenated organic compound means one or more organic compoundscontaining two to six carbon atoms selected from the group of aliphaticcarboxylic acids and salts, alkanols and alkoxide salts, and aldehydes.Often oxygenated organic compound is a mixture of organic compoundsproduced by the microorganisms contained in the aqueous fermentationbroth.

The terms selectively reducing the concentration of hydrogen cyanide andselective oxidation of hydrogen cyanide refer to selectivity in respectof carbon monoxide. The concentration of other components in the syngascan be reduced and the other components oxidized to the same, greater orlesser extent than that of hydrogen cyanide. A negligible amount ofcarbon monoxide can be oxidized when selectively reducing the hydrogencyanide concentration, and preferably less than about 0.5, morepreferably less than about 0.2, percent of the carbon monoxide isoxidized.

A soluble permanganate anion means that in the aqueous solution, thepermanganate anion is soluble in the concentration present and under theconditions of the contacting with the gas feed. The extent of solubilitycan be dependent upon cations present and the temperature of the aqueoussolution. Concentrations of permanganate anion in the aqueous solutionare based upon the mass of the anion itself and does not include anywater of hydration.

Syngas is a carbon monoxide-containing gas which preferably containshydrogen and optionally contains carbon dioxide, and the total carbonmonoxide and hydrogen Component Composition is at least about 50percent.

Gas Feed

The gas feed used in the processes of this invention contains at leastabout 5 volume percent carbon monoxide and contains between about 1 and500, and preferably between about 1 and 50 or 100, parts per million byvolume hydrogen cyanide. The feed gas may contain other components andmay be a syngas. Examples of other components include those that can bein a significant concentration, e.g., each being greater than 1 percentby volume such as hydrogen, carbon dioxide, nitrogen, and water vapor.Depending upon the source of the gas feed, it can contain lighthydrocarbons such as methane and ethane that may be present in amountsgreater than 1 percent by volume. Other components that may be present,typically in amounts less than 1 percent by volume include, but are notlimited to, acetylene, ethylene, propylene, ammonia, nitric oxide,hydrogen sulfide, carbonyl sulfide, benzene, toluene, xylene,trimethylbenzene, cumene, and tars and naphthalene. The processes ofthis invention are particularly useful for the oxidation of acetylene,nitric oxide and hydrogen sulfide in addition to the oxidation ofhydrogen cyanide.

Syngas is one source of such a gas substrate. Syngas can be made frommany carbonaceous feedstocks. These include sources of hydrocarbons suchas natural gas, biogas, biomass, especially woody biomass, gas generatedby reforming hydrocarbon-containing materials, peat, petroleum coke,coal, waste material such as debris from construction and demolition,municipal solid waste, and landfill gas. Syngas is typically produced bya gasifier. Any of the aforementioned biomass sources are suitable forproducing syngas. The syngas produced thereby will typically containfrom 10 to 60 mole % CO, from 10 to 25 mole % CO₂ and from 10 to 75,often at least about 30, and preferably between about 35 and 70 or 75,mole % H₂. The syngas may also contain N₂ and CH₄ as well as tracecomponents such as H₂S and COS, NH₃ and HCN. Other sources of the gassubstrate include gases generated during petroleum and petrochemicalprocessing. These gases may have substantially different compositionsthan typical syngas, and may be essentially pure hydrogen or essentiallypure carbon monoxide. The gas substrate may be obtained directly fromgasification or from petroleum and petrochemical processing or may beobtained by blending two or more streams. For the sake of ease ofreading, the term syngas will be used herein and will be intended toinclude these other gas substrates.

The gas feed may be treated to remove or alter the compositionincluding, but not limited to, removing components by chemical orphysical sorption, membrane separation, and selective reaction. Oneoptional cleanup operation is water scrubbing which may be conducted inthe presence of a reactant. See, for instance, United States PublishedPatent Application No. 20110097701 A1, hereby incorporated by referencein its entirety. The water scrubbing also serves to remove at least aportion of other impurities from the syngas such as ethylene, acetylene,ammonia, hydrogen sulfide and carbonyl sulfide. The scrubbing may beconducted in any convenient manner. Often, the temperature of thescrubbing is in the range of about 4° C. to 50° C., and the scrubbingmay be conducted at subatmospheric, atmospheric or superatmosphericpressure, e.g., frequently at about 105 to 1000 KPa absolute. Waterpressure swing absorption can be used if desired. The pH of thescrubbing solution is usually maintained in the range of about 5.5 to 8,preferably between about 6 to 6.5. To the extent that oxidizablecomponents such as, but not limited to, hydrogen cyanide and acetylene,are removed by any such treatment, the load on the permanganateoxidation is reduced thereby reducing the amount of permanganate anionrequired per unit volume of gas treated, and also, the control of theselective oxidation can be enhanced.

Aqueous Solution

The processes of this invention use an aqueous solution containingpermanganate anion to selectively oxidize hydrogen cyanide. Thepermanganate anion may be supplied by any suitable water soluble salt ofpermanganate, and due to availability and water solubility, sodiumpermanganate and potassium permanganate are preferred sources ofpermanganate anion. As stated above, the concentration of permanganateanion is maintained in the aqueous solution in an amount less than about2000 parts per million by mass. In general, lower concentrations ofpermanganate anion are preferred both because lower concentrationsfacilitate modulation of the selective oxidation and becausepermanganate is subject to degradation, particularly the presence ofmagnesium dioxide. The absolute amount of permanganate anion lost due todegradation is reduced with lower permanganate concentrations in theaqueous solution. The aqueous solution is preferably devoid of anyundissolved permanganate compound.

During the contacting with the gas feed, permanganate anion is primarilyreduced to manganese dioxide although some intermediate reduced statesof manganese species may exist in the aqueous solution. Accordingly, inmost instances, the concentration of permanganate in the aqueoussolution will decrease with increasing contact time with the gas feed.The concentrations provided herein are intended to be initialconcentrations of permanganate anion. Preferably sufficient permanganateanion is provided in the aqueous solution such that a residualconcentration of permanganate anion is retained in the aqueous solutionupon completion of the contact with the gas feed. This is particularlythe case where it is desired to operate the process in a hydrogencyanide mass transfer of limited mode. Often, the aqueous solutioncontains between about 100 and 750 parts by million by mass ofpermanganate anion, and the spent aqueous solution contains at leastabout 10, preferably at least about 50, parts by million by mass ofpermanganate anion.

The aqueous medium may contain additives to control pH. Other additivesinclude those that can facilitate gas-liquid contact and mass transferand those that can assist in the reduction of components other thanhydrogen cyanide in the gas feed.

Conditions of Contacting Gas Feed with Aqueous Solution

The contact between the gas feed and aqueous solution is conducted underconditions sufficient for permanganate anion to oxidize hydrogen cyanideand generate manganese dioxide. The conditions provide a treated gashaving a concentration of hydrogen cyanide that is less than about 30percent of that in the gas feed, and preferably a treated gas containsless than about 1, more preferably less than about 0.5, and mostpreferably less than about 0.1, part per million by volume hydrogencyanide. Under these conditions, other components in the gas feed, ifpresent, such as acetylene, nitric oxide, ethylene, and hydrogen sulfideare also oxidized by the permanganate anion. In many instances, thepercentage reduction of the concentration of acetylene, nitric oxide andhydrogen sulfide from the gas feed to the treated gas roughlyapproximates the reduction of hydrogen sulfide concentration.Accordingly, in situations where the gas feed contains one or more ofacetylene, nitric oxide and hydrogen sulfide which are sought to beremoved, monitoring the reduction in concentration of hydrogen cyanidefrequently provides the operator in indirect indication of theeffectiveness of reduction of the concentration of these othercomponents.

The temperature at which the syngas and aqueous solution are contactedis between about 4° C. and 50° C., preferably between about 10° and 40°C. although the process can be operated at a temperature within a widerange, and is frequently used as a variable to modulate the process. Formost gases having limited solubility in water, including, but notlimited to, hydrogen cyanide, carbon monoxide, hydrogen, acetylene, andnitric oxide, the solubility decreases with increasing temperature.Temperature also is a factor in the kinetic rate of oxidation of acomponent by permanganate as well as degradation of permanganate. Withincreasing temperature, the reaction rate increases. Hence, the operatoris able to select a temperature which provides a combination of rate ofmass transfer and reaction rate that can modulate the processes of thisinvention. For example, where a mass transfer-limited mode of operationis sought, higher temperatures within the above range are generallyused. However, if the temperature becomes too high, undue oxidation ofcarbon monoxide may occur and the rate of degradation of permanganatemay be increased to an economically unacceptable level. Also, if thetemperature is too high, the solubility of hydrogen cyanide in theaqueous solution is decreased, and thus may adversely affect the rate ofmass transfer of hydrogen cyanide to the aqueous solution therebyrequiring greater durations of contact in order to obtain the soughtreduction in hydrogen cyanide.

The processes of this invention can use a gas feed pressure fallingwithin a wide range. Although pressure can affect the rate of masstransfer of hydrogen cyanide to the aqueous solution, the economicviability of the processes of this invention is not dependent uponincreasing the pressure of the gas feed from that at which it issupplied. Accordingly, the processes of this invention are suitable fortreating gas feed supplied at a pressure less than about 2000 kPaabsolute. Often the pressure is between about 105 and 1500 kPa absolute,and some instances the pressure can be in the range of about 110 to 150kPa absolute.

The duration of contact between the gas feed and the aqueous solution issufficient to provide a desired mass transfer of hydrogen cyanide andother oxidizable components from the gas phase to the aqueous phase. Themethod of contact between the gas feed and aqueous solution affects themass transfer of hydrogen cyanide, and thus the duration of contact willbe dependent upon the method used for securing the gas liquid contact.Any suitable method for securing the contact between the gas feed andaqueous solution can be used in the processes of this invention.Examples of such contact methods include the use of bubble columns,liquid spray columns, and trayed and packed (structures or random)columns which increase the surface area between the gas feed and aqueoussolution per unit volume of aqueous solution. Particularly attractiveapparatus for use in the processes this invention provide very lowpressure drops between a pressure of the gas feed to be contacted withthe aqueous solution and the pressure of the treated gas after contactwith the aqueous solution. Preferably the pressure of the treated gas iswithin 50 kPa below the pressure of the gas feed passed to theabsorption vessel.

The aqueous solution may be used in a semi-batch manner, i.e., the gasfeed is passed through the aqueous solution as in a bubble columnabsorber, or may be flowing as in a countercurrent, co-current, or crosscurrent absorber. The concentration of permanganate anion may bemaintained by continuously or intermittently adding permanganate anion.A preferred mode of operation is using a flowing, aqueous solution thatpasses countercurrent to the flow of the gas feed.

The average residence time of the gas feed in contact with the aqueoussolution is sufficient to enable the desired reduction in theconcentration of hydrogen cyanide in the gas feed. Often the averageresidence time based upon the superficial gas velocity is less thanabout 30, preferably less than about 10, seconds providing good masstransfer exist between the gas phase and aqueous solution. As discussedabove operation in a mass transfer-limited mode facilitates the masstransfer of hydrogen cyanide to the aqueous solution which enablesrelatively low average residence time to be achieved. The masstransfer-limited mode of operation also enables the treated gas tocontain very low concentrations of hydrogen cyanide, includingconcentrations less than about 0.5, especially less than about 0.1,parts per million by volume.

Preferably, the pH of the aqueous solution is maintained above about 5,preferably 5.5 to 8, preferably between about 6 to 6.5, to reduce therate of decomposition of permanganate.

In preferred modes of operation of the processes of this invention thereaction density per unit time is controlled by a combination of themass transfer rate from the gas feed of hydrogen cyanide and otheroxidizable components to the aqueous solution and the rate of oxidationof permanganate anion in the aqueous solution. The rate of oxidation ofpermanganate anion is determined by the oxidation of components from thefeed gas as well as the autocatalytic degradation of permanganate.Frequently less than about 100, and preferably less than about 75, partsper million by mass of permanganate anion of aqueous solution isconverted to manganese dioxide per second.

In the preferred operations of the processes of this invention aqueoussolution is continuously added and withdrawn from the absorption zone inorder to remove manganese dioxide from the aqueous solution. Manganesedioxide is believed to promote the degradation of permanganate anion.All, or a portion of, the withdrawn aqueous solution may be treated byany suitable solids separation operation including, but not limited to,filtration, centrifugation, and settling vessels to remove manganesedioxide. Frequently, the concentration of manganese dioxide in theaqueous solution contacting the gas feed is maintained less than about250 parts per million by mass, and the mole ratio of permanganate anionto manganese dioxide is preferably greater than about 1:1, preferablygreater than about 10:1. If the entire aqueous solution is not subjectto the solid separation operation to remove manganese dioxide, theportion subjected to the solid separation operation should be sufficientto maintain the sought steady-state concentration of manganese dioxidein the recycled aqueous solution. Thus, the portion subjected to thesolid separation operation can vary widely depending upon the rate offormation of manganese dioxide and the sought steady-state concentrationof manganese dioxide in the recycled aqueous solution. Often, betweenabout 5 and 50, say, between about 10 and 35 percent of the aqueoussolution is subjected to the solid separation operation.

Makeup permanganate anion and water can be added to the absorption zoneintermittently or continuously in order to maintain a steady-stateoperation. The makeup may be introduced into any recycle stream ofaqueous solution or may be directly added to the aqueous solution in theabsorption zone.

In some embodiments of the processes of this invention where the aqueoussolution is continuously withdrawn from the contact with the gas feed,the aqueous solution withdrawn contains unreacted, dissolvedpermanganate anion. In this manner, the presence of permanganate anionenables the oxidation of hydrogen cyanide and other oxidizablecomponents that have been removed from the gas feed to be oxidizedwhether it be during the time of contact between the gas feed andaqueous solution or after the aqueous solution has been withdrawn fromthe absorption zone. Hence, beneficial driving forces can be maintainedto facilitate mass transfer of hydrogen cyanide from the gas phase andthe lower the concentration of hydrogen cyanide in the treated gas, andthe time elapsed after withdrawal of the aqueous solution from theabsorption zone enables further oxidation of hydrogen cyanide and otheroxidizable components to occur. In many instances, the concentration ofpermanganate anion in the withdrawn aqueous solution is at least about50, preferably at least about 100, parts per million by mass of aqueoussolution.

Use of Treated Gas

the treated gas provided by the processes of this invention may be usedfor any suitable purpose including, but not limited to, Fischer Tropschprocesses; chemical syntheses including, but not limited to, thesynthesis of methanol; and as a substrate for bioconversion to abioproduct.

As the processes of this invention serve to remove not only hydrogencyanide but also acetylene and other components that can adverselyaffect the microorganisms, the treated gas is particularly useful for afeed to anaerobic bioconversion processes.

Anaerobic bioconversion processes include the production of oxygenatedorganic compounds. The oxygenated organic compound produced will dependupon the microorganism used for the fermentation and the conditions ofthe fermentation. Bioconversions of CO and H₂/CO₂ to acetic acid,n-butanol, butyric acid, ethanol and other products are well known. Forexample, a concise description of biochemical pathways and energetics ofsuch bioconversions have been summarized by Das, A. and L. G. Ljungdahl,Electron Transport System in Acetogens and by Drake, H. L. and K. Kusel,Diverse Physiologic Potential of Acetogens, appearing respectively asChapters 14 and 13 of Biochemistry and Physiology of Anaerobic Bacteria,L. G. Ljungdahl eds., Springer (2003). Any suitable microorganisms thathave the ability to convert the syngas components: CO, H₂, CO₂individually or in combination with each other or with other componentsthat are typically present in syngas may be utilized. Suitablemicroorganisms and/or growth conditions may include those disclosed inU.S. patent application Ser. No. 11/441,392, filed May 25, 2006,entitled “Indirect Or Direct Fermentation of Biomass to Fuel Alcohol,”(U.S. Published Patent Application No. 2007/0275447) which discloses abiologically pure culture of the microorganism Clostridiumcarboxidivorans having all of the identifying characteristics of ATCCno. BAA-624; U.S. Pat. No. 7,704,723 entitled “Isolation andCharacterization of Novel Clostridial Species,” which discloses abiologically pure culture of the microorganism Clostridium ragsdaleihaving all of the identifying characteristics of ATCC No. BAA-622; bothof which are incorporated herein by reference in their entirety.Clostridium carboxidivorans may be used, for example, to ferment syngasto ethanol and/or n-butanol. Clostridium ragsdalei may be used, forexample, to ferment syngas to ethanol.

Suitable microorganisms and growth conditions include the anaerobicbacteria Butyribacterium methylotrophicum, having the identifyingcharacteristics of ATCC 33266 which can be adapted to CO and used andthis will enable the production of n-butanol as well as butyric acid astaught in the references: “Evidence for Production of n-Butanol fromCarbon Monoxide by Butyribacterium methylotrophicum,” Journal ofFermentation and Bioengineering, vol. 72, 1991, p. 58-60; “Production ofbutanol and ethanol from synthesis gas via fermentation,” FUEL, vol. 70,May 1991, p. 615-619. Other suitable microorganisms include: ClostridiumLjungdahlii, with strains having the identifying characteristics of ATCC49587 (U.S. Pat. No. 5,173,429) and ATCC 55988 and 55989 (U.S. Pat. No.6,136,577) that will enable the production of ethanol as well as aceticacid; Clostridium autoethanogemum sp. nov., an anaerobic bacterium thatproduces ethanol from carbon monoxide. Jamal Abrini, Henry Naveau,Edomond-Jacques Nyns, Arch Microbiol., 1994, 345-351; Archives ofMicrobiology 1994, 161: 345-351; and Clostridium Coskatii having theidentifying characteristics of ATCC No. PTA-10522 described in U.S.Published Patent Application No. 2011/0229947. All of these referencesare incorporated herein in their entirety.

The aqueous fermentation broth will comprise an aqueous suspension ofmicroorganisms and various media supplements. Suitable microorganismsgenerally live and grow under anaerobic conditions, meaning thatdissolved oxygen is essentially absent from the fermentation liquid. Thevarious adjuvants to the aqueous fermentation broth may comprisebuffering agents, trace metals, vitamins, salts etc. Adjustments in themenstruum may induce different conditions at different times such asgrowth and non-growth conditions which will affect the productivity ofthe microorganisms. Previously referenced U.S. Pat. No. 7,704,723discloses the conditions and contents of suitable aqueous fermentationbroth for bioconversion CO and H₂/CO₂ using anaerobic microorganisms.

The fermentation broth is maintained under anaerobic fermentationconditions including a suitable temperature, say, between 25° and 60°C., frequently in the range of about 30° to 40° C. The conditions offermentation, including the density of microorganisms, aqueousfermentation broth composition, and fermentation zone depth, arepreferably sufficient to achieve the sought conversion efficiency ofhydrogen and carbon monoxide.

The fermentation conditions are preferably sufficient to effect at leastabout 85, preferably at least about 90, percent of the hydrogen in thesubstrate gas fed to the bioreactor to oxygenated organic compound. Asstated above, a combination of bubble size and duration of contact withthe aqueous fermentation menstruum are necessary to achieve these highconversions. However, the ease and ability to achieve these highconversions is also dependent upon having the specified electron tocarbon ratios and carbon dioxide partial pressures in the substratedepleted gas phase. For commercial operations, the fermentationoperation preferably provides a total molar conversion of hydrogen andcarbon monoxide in the feed gas in the range of at least about 93,preferably at least about 97, mole percent. If required to provideadequate contact time between the gas bubbles and the aqueousfermentation menstruum, more than one bioreactor may be used in gas flowseries in the bioreactor. The use of sequential, deep tank bubble columnbioreactors is disclosed in U.S. patent application Ser. No. 13/243,062,filed on Sep. 23, 2011, herein incorporated by reference in itsentirety.

Drawing

A general understanding of the invention and its application may befacilitated by reference to FIG. 1 but is not in limitation of theinvention. FIG. 1 is a schematic depiction of an apparatus generallydesignated as 100 suitable for practicing the processes of thisinvention. FIG. 1 includes an optional water pressure swing absorber totreat the syngas prior to selective oxidation. FIG. 1 omits minorequipment such as pumps, compressors, valves, instruments and otherdevices the placement of which and operation thereof are well known tothose practiced in chemical engineering. FIG. 1 also omits ancillaryunit operations.

As shown, syngas is passed via line 102 to absorption: 104 of a pressureswing absorption unit. Water at a temperature of about 0° C. to about25° C., preferably between about 4° C. and 15° C., and for the purposeof this description about 7° C. is provided to absorption column 104 vialine 114. Typically lower temperatures are used to minimize the amountsof carbon monoxide and hydrogen absorbed in the water. The water, ifdesired, can contain other components to assist in the removal ofimpurities from syngas. These other components include buffers andreactants such as aldehydes, hypochlorites, peroxygenates, and the like.Absorption column 104 is preferably at a pressure of between about 50and 1500, say between about 200 and 1,000, kPa gauge. Absorption column104 may be of any convenient absorption column design including, but notlimited to, spray columns, packed and trayed columns, and bubblecolumns. Preferably, absorption column 104 is of a design that posesminimal pressure drop loss to the syngas. The residence time in theabsorption column should be sufficient to provide a desired reduction inimpurities in the syngas. When used, the pressure swing absorption unitis generally employed to reduce the hydrogen cyanide concentration inthe syngas to below about 10, preferably below about 5, parts permillion by volume. By reducing the hydrogen cyanide concentration, theamount of oxidant required is reduced thereby enhancing the economics ofthe process.

Spent water exits absorber column 104 via line 106. The spent water ispassed to desorption column 110. Desorption column 110 is maintained ata lower pressure than absorption column 104, and is usually a pressurein the range of between about 5 and 200, preferably between about 50 and110, kPa absolute. The lower pressure should be sufficient to remove atleast hydrogen cyanide from the water to maintain a steady-stateoperation. The temperature of the water may be the same or higher orlower than the temperature of the water in absorption column 104. Makeupwater is provided to desorption column via line 108. It is understoodthat the makeup water may be provided either to the desorption column110 or to absorption column 104.

Desorption gases exit desorption column 110 via line 112. Line 114returns the water for absorption to absorption column 104. A purgestream is removed from line 114 via line 116.

Syngas exits absorber column 104 via line 118 and is passed topermanganate oxidizer 120. For purpose of discussion in conjunction withthis FIGURE, permanganate oxidizer 120 is a packed column containingpacking 122 and has a lower conical section to facilitate the removal ofmanganese dioxide from the vessel. Treated syngas is withdrawn frompermanganate oxidizer 120 by line 124. In permanganate oxidizer 120 thesyngas is contacted with an aqueous solution of about 500 parts permillion by mass of sodium permanganate. The contacting is at atemperature of about 38° C. and at a pressure substantially that of thesyngas exiting absorber column 104. The aqueous solution is supplied atthe top of permanganate oxidizer 120 and is withdrawn at the bottom vialine 126.

The withdrawn aqueous solution contains manganese dioxide and unreactedsodium permanganate. All, or a portion of, the aqueous solution in line126 can be passed to solids separator 128. In solids separator 128,manganese dioxide is removed. As shown, a manganese dioxide-containingstream is withdrawn from solids separator 128 via line 130, andsupernatant aqueous solution having a reduced concentration of manganesedioxide is passed via line 132 as a recycle to permanganate oxidizer120. Returning to line 126, the portion of the aqueous solution notpassed to the solid separator 128 is recycled to permanganate oxidizer120 via line 134. This portion of the aqueous solution is combined withthe supernatant aqueous solution in line 132. Makeup permanganatesolution is also combined with the aqueous solution being recycled inline 134 to permanganate oxidizer 120. This makeup permanganate solutionis supplied via line 136 to line 134 and is a more concentrated solutionof sodium permanganate sufficient to compensate for water losses vialine 130 as well as replace the reacted permanganate to maintain thesought concentration of sodium permanganate in the aqueous solutionpassing to the top of permanganate oxidizer 120.

EXAMPLES

For purposes of illustration of the invention and not in limitationthereof the following examples based upon computer simulations areprovided. All parts and percentages of solids are by mass and all partsand percentages of liquids and gases are by volume unless otherwisestated.

A permanganate oxidizer such as described in connection with the FIGUREis used to treat a syngas of the composition set forth in Table I and asyngas of the composition set forth in Table II below. The aqueoussolution to the top of the permanganate oxidizer contains about 250parts per million by mass of sodium permanganate. The permanganateoxidizer contains Beta Ring™ random packing having a nominal size of 2inches from Koch-Glitsch, Wichita, Kans. The syngas is passed throughthe permanganate oxidizer to provide a residence time based onsuperficial velocity of about 2.7 seconds, and the aqueous solution isprovided at a rate of about 0.3 liters per second per square meter ofpermanganate oxidizer diameter. The compositions of the treated syngasare set forth in Tables I and II.

TABLE I Syngas after Component Syngas treatment H2 40.6% v 40.6% v CO44.8% v 44.8% v CO2 10.4% v 10.3% v C2H2 8.5 ppmv 0.8 ppmv C2H4 22.9ppmv 6.9 ppmv H2S 72.7 ppmv 3.6 ppmv COS 1.2 ppmv 1.2 ppmv NH3 57.8 ppmv0.6 ppmv NO 13.1 ppmv 5.2 ppmv HCN 5.7 ppmv 0.1 ppmv Other balancebalance

TABLE II Syngas after Component Syngas treatment H2 57.7% v 58.4% v CO34.8% v 35.2% v CO2 2.4% v 2.4% v C2H2 5.0 ppmv 0.5 ppmv C2H4 15.0 ppmv4.6 ppmv H2S 0.0 ppmv 0.0 ppmv NO 5.0 ppmv 2.0 ppmv HCN 50.0 ppmv 0.1ppmv H2O 2.6% v 1.5% v Other balance balance

As can be seen from the above Tables, virtually all the hydrogen cyanideis selectively oxidized to about 0.1 parts per million by volume, orwithout adversely affecting the carbon monoxide and hydrogen content ofthe syngases. The permanganate oxidation also oxidized acetylene,ethylene, nitric oxide and hydrogen sulfide.

It is claimed:
 1. A process for selectively reducing the concentrationof hydrogen cyanide with respect to carbon monoxide in a gas feed, saidgas feed containing carbon monoxide and hydrogen cyanide, comprising: a.continuously passing said gas feed into contact with an aqueous solutionof permanganate anion under conditions sufficient for the permanganateanion to oxidize the hydrogen cyanide and generate manganese dioxide,the duration of said contacting being at least sufficient to provide atreated gas having a concentration of the hydrogen cyanide that is lessthan that in the gas feed; b. continuously removing said treated gasfrom contact with the aqueous solution; c. intermittently orcontinuously replenishing permanganate anion to said aqueous solution;and d. intermittently or continuously removing manganese dioxide fromsaid aqueous solution.
 2. The process of claim 1 wherein the treated gascontains less than about 0.5 parts per million by volume of hydrogencyanide.
 3. The process of claim 2 wherein the treated gas contains lessthan about 0.1 parts per million by volume of hydrogen cyanide.
 4. Theprocess of claim 2 wherein the gas feed contains between about 1 and 50parts per million by volume of hydrogen cyanide.
 5. The process of claim1 wherein the aqueous solution contains less than about 1000 parts permillion by mass of permanganate anion.
 6. The process of claim 1 whereinthe average duration of contact between the gas feed and aqueoussolution is less than about 30 seconds.
 7. The process of claim 1wherein the average duration of contact between the gas feed and aqueoussolution is less than about 10 seconds.
 8. The process of claim 1wherein in step a less than about 100 parts per million by mass ofpermanganate anion in the aqueous solution are converted to manganesedioxide per second.
 9. The process of claim 1 wherein in step a lessthan about 75 parts per million by mass of permanganate anion in theaqueous solution are converted to manganese dioxide per second.
 10. Theprocess of claim 1 wherein the contact between the gas feed and aqueoussolution is conducted in an absorption vessel, and aqueous solution iscontinuously added and withdrawn from said absorption vessel.
 11. Theprocess of claim 10 wherein the contact between the gas feed and aqueoussolution is countercurrent.
 12. The process of claim 11 wherein theabsorption vessel contains structured packing.
 13. The process of claim12 wherein the pressure of the treated gas is within 50 kPa below thepressure of the gas feed passed to the absorption vessel.
 14. Theprocess of claim 11 wherein at least a portion of the aqueous solutionwithdrawn from said absorption vessel is subjected to solids removalunit operation to remove manganese dioxide and is recycled to theabsorption vessel.
 15. The process of claim 14 wherein the solidsremoval unit operation comprises a centrifuge.
 16. The process of claim1 wherein the gas feed comprises at least one other impurity selectedfrom the group of acetylene, nitric oxide and hydrogen sulfide, and atleast a portion of said at least one other impurity is oxidized bypermanganate anion during the contact of step (a).
 17. The process ofclaim 1 wherein the oxidation of hydrogen cyanide in step a is masstransfer limited.
 18. The process of claim 1 wherein the gas feedcomprises 10 to 60 mole percent carbon monoxide.
 19. The process ofclaim 1 wherein the duration of said contacting is at least sufficientto provide the treated gas having a concentration of the hydrogencyanide that is about 30 percent less than that in the gas feed.
 20. Theprocess of claim 1 wherein: said gas feed contains at least about 5volume percent carbon monoxide and between about 1 and 500 parts permillion by volume hydrogen cyanide; said aqueous solution containsbetween about 50 and 2000 parts per million by mass of permanganateanion: said contacting is at a temperature of between about 4° C. and50° C.; and said gas feed is at a pressure less than about 2000 kPaabsolute.